The interdiction of illicit drugs, explosives, and other contraband is an important goal of law enforcement. To that end, a variety of technologies have been developed and deployed for the non-intrusive inspection of containers not readily susceptible to visual scrutiny from the outside. The non-intrusive aspect of these inspection techniques is important; the great majority of containers do not carry contraband, and the public would not long tolerate the delays, disruption (and in some cases damage) of property, and invasions of privacy that would occur if invasive inspection means were commonly used. Non-intrusive inspection is typically non-destructive and can usually be accomplished faster than intrusive inspection, thereby increasing productivity of inspectors. Increased productivity means more containers inspected and more contraband interdicted.
Among non-intrusive inspection methods, x-ray imaging in its many forms has been a proven technology capable of detecting a variety of contraband. X-ray systems have been based on transmission imaging in any of a variety of implementations: cone-beam (fluoroscopes), fanbeam, flying-spot, multi-projection configurations; dual-energy imaging; computed tomography; as well as on imaging incorporating the detection of x-ray radiation scattered in various directions.
With only a few exceptions, x-ray imaging systems for contraband detection have operated within the source-energy range of 70 keV to 160 keV. Since the penetrating power of these sources is limited, inspection is limited to relatively small parcels and containers. A few large, expensive, high-energy transmission imaging systems have also been built for the inspection of large containers or vehicles. These systems typically operate within the range of 6 MeV to 12 MeV, require heavily-shielded, fixed installations, and provide no means to distinguish organic materials.
Radiant energy imaging with a scanning pencil beam is the subject of U.S. Pat. No. 3,780,291. The creation and use of images from scattered x-radiation in conjunction with direct transmission images is the subject of U.S. Pat. No. 5,313,511. These patents are hereby incorporated herein by reference. Neither of the foregoing references, nor any prior art known to the inventors herein, discloses or suggests how x-ray images, in a sense defined below, may be formed of cargo containers on the scale of motor vehicles or railroad cars using a source of penetrating radiation and detectors mounted on a mobile platform.